Coking is a carbon rejection process that is commonly used for upgrading of heavy oil feeds and/or feeds that are challenging to process, such as feeds with a low ratio of hydrogen to carbon. In addition to producing a variety of liquid products, typical coking processes can also generate a substantial amount of coke. Because the coke contains carbon, the coke is potentially a source of additional valuable products in a refinery setting. However, fully realizing this potential remains an ongoing challenge.
Coking processes in modem refinery settings can typically be categorized as delayed coking or fluidized bed coking. Fluidized bed coking is a petroleum refining process in which heavy petroleum feeds, typically the non-distillable residues (resids) from the fractionation of heavy oils are converted to lighter, more useful products by thermal decomposition (coking) at elevated reaction temperatures, typically about 480° C. to 590° C., (about 900° F. to 1100° F.) and in most cases from 500° C. to 550° C. (about 930° F. to 1020° F.). Heavy oils which may be processed by the fluid coking process include heavy atmospheric resids, petroleum vacuum distillation bottoms, aromatic extracts, asphalts, and bitumens from tar sands, tar pits and pitch lakes of Canada (Athabasca, Alta.), Trinidad, Southern California (La Brea (Los Angeles), McKittrick (Bakersfield, Calif.), Carpinteria (Santa Barbara County, Calif.), Lake Bermudez (Venezuela) and similar deposits such as those found in Texas, Peru, Iran, Russia and Poland.
Flexicoking process is described in patents of Exxon Research and Engineering Company, including, for example, U.S. Pat. No. 3,661,543 (Saxton), U.S. Pat. No. 3,759,676 (Lahn), U.S. Pat. No. 3,816,084 (Moser), U.S. Pat. No. 3,702,516 (Luckenbach), U.S. Pat. No. 4,269,696 (Metrailer). A variant is described in U.S. Pat. No. 4,213,848 (Saxton) which introduces a stream of light hydrocarbons from the product fractionator into the reactor instead of the stream of hot coke particles from the heater. Another variant is described in U.S. Pat. No. 5,472,596 (Kerby).
One of the difficulties with operating a fluidized coker is selecting a desirable temperature profile for both the coking section and the stripping section of the reactor. Conventionally, hot coke from a heater or gasifier is usually returned to the coking section to provide at least part of the heat for the coking process. Due in part to the endothermic nature of both the coking and the stripping process, the stripping section in a conventional reactor typically operates at a lower temperature than the coking section. This is often the reverse of what would be desirable from a yield standpoint. The yield of liquid products from the coking section tends to increase as the coking temperature is decreased. However, due to the need to reduce or minimize coking within the stripping section, the temperature of the coking section is typically selected to provide a sufficiently high temperature in the stripping section.
U.S. Pat. No. 9,670,417 provides an example of a configuration designed to allow a stripping section of a fluidized coking reactor to operate at a lower temperature than the coking section. Due to substantial recirculation from the stripping zone to the coking zone, simply providing heated particles to the stripping zone can tend to result in heating of both the coking zone and the stripping zone. The configuration in U.S. Pat. No. 9,670,417 provides an annular baffle between the coking and stripping zone to reduce or minimize this mixing. In combination with the additional piping to provide heated coke particles to both the coking zone and the stripping zone, this configuration is described as being suitable for having a stripping zone temperature that is greater than the coking zone temperature.
It would be desirable to have methods for separately controlling the temperature in the stripping zone and the coking zone. Additionally, it would be desirable for such methods to allow for operating the stripping zone at a higher temperature than the coking zone while still achieving a yield benefit for the liquid products generated from the fluidized coking system.